This invention relates to improved, cation-particle-coated filter aid materials for filtration application. In particular it relates to filter aid materials coated with anion exchange resins having particle sizes of 0.05 to 1 micrometer in diameter, electrostatically bound to the surface of the filter aid material.
Filter aids are well-known in the filtration art. They are solid materials, insoluble in the liquid to be filtered, which are added to the liquid or are coated upon a filter or filter support, their purpose being to speed filtration, reduce fouling of the filter surface, reduce cracking of the filter layer, or otherwise to improve filtration characteristics. Materials which are frequently used as filter aids include cellulose fibers, diatomaceous earth, charcoal, expanded perlite, asbestos fibers and the like.
Filter aids are often treated to improve their effect upon the filter. They have been treated with soluble polyelectrolytes to change their surface characteristics, as described by Halbfoster in U.S. Pat. No. 4,190,532. They also have been modified chemically to give them ion exchange properties, as described by Kressman in British Pat. No. 1,123,908, or mixed with ground ion exchange resins having particle sizes of about 40 to 250 micrometers, and the resulting mixture used as a filter aid, as described by Halbfoster, above, or Kunin et al., U.S. Pat. No. 4,187,120, or in British Pat. No. 1,455,692.
Filters are often prepared from flocculated mixtures of cation and anion exchange resins, either ground, conventional resins as taught in U.S. Pat. Nos. 3,250,702 and 3,250,703, or from emulsion polymer ion exchange resins, as taught in U.S. Pat. No. 4,200,695. These flocs are coated on filter supports, and are used for removal of colloidal and dissolved solids from water and other aqueous process streams. A major problem of filters using these filter media is a tendency for the filter layer to crack during use, which leads to particulate leakage and fouling of the filter support. Fibrous filter-aid material deposited on top of the filter layer significantly aids in the prevention of such cracks, thereby protecting the filter support and prolonging the useful life of the filter layer.
We have now discovered that cationic particles bearing anion exchange functional groups, and having a particle size of from about 0.05 to about 1 micrometer in diameter, coated upon the surface of conventional filter aid materials, produces a cation-particle-coated filter aid material which is superior for many applications. Such cationic particles bind themselves electrostatically to the negatively charged surface which forms on filter aid materials when they are wet with water. The electrostatic bond thus formed is sufficiently strong to resist resin-particle separation from the surface of the filter aid material during rinsing or other treatment, this strong bonding being in marked contrast to the behavior of larger charged particles in contact with filter aid materials, as described in the above references. The particles, once bound, are essentially irremovable from the filter aid surface.
As used herein, the term "cation-particle-coated filter aid" refers to the materials of the present invention, bearing a surface coating of the fine cationic particles described above bound tightly to the filter aid by the electrostatic charges. The cationic particles themselves are also known as anion exchange resins, and are designated herein interchangeably as cationic particles and as anion exchange resins, because they possess not only a cationic charge, but anion exchange functional groups in their insoluble polymer structure.
Cation-particle-coated filter aids possess a high surface area of active anion-exchange sites, for wherever the small cationic particles of anion resins are not in contact with the filter aid they are available for both adsorption and anion exchange. Surprisingly, cation-particle-coated filter aid materials permit a significant increase in the total filtration capacity of a filter comprising an ion exchange resin floc and a coating of cation-particle-coated filter aid over such a filter overcoated with conventional filter aids.
The conventional filter aids useful in preparing the materials of the present invention are well-known to those familiar with the filtration art. They include, but are not limited to, cellulose fibers, including those cellulose fibers which have been variously treated with commercial surface treatments, asbestos fibers, polyacrylonitrile fibers, charcoal, diatomaceous earth and expanded perlite. The cation particles of this invention are those particles bearing anion exchange functional groups and having a very fine particle size; the preferred particle size is from about 0.05 to about 1 micrometer, and a more preferred particle size is from about 0.05 to about 0.2 micrometers. Smaller particle sizes encourage tighter bonding between the anion exchange resin particles and the surface of the filter aid materials. Strongly basic resins in this particle size range are preferred, and more preferred are those resins functionalized with quaternary amine functional groups.
The improved, cation-particle-coated filter aid materials of the present invention are prepared by mixing a dispersion of the resin particles in water with a slurry of the filter aid material in water. As these materials are mixed, the positively charged resin particles bind electrostatically to the negatively charged surface of the filter aid material until the surface is saturated. This saturation typically occurs when about 2% or less of the resin, based on the dry weight of the filter aid material, has bound itself to the filter aid surface. Until the surface has been saturated, the supernatant liquid in the mixture is clear; at saturation the excess resin particles remain free, and the supernatant liquid is cloudy. Once the materials have been combined, the cation-particle-coated filter aid may be rinsed to remove excess unbound resin, with no loss of the bound resin from the surface of the filter aid.
Once prepared, the cation-particle-coated filter aid materials of the present invention may be used for purposes similar to those of untreated filter aids; such uses are well-known in the art. Conventional flow rates and levels of application may be used as guides for determining the optimum conditions for a particular application.
Applicants do not wish to be bound to any one theory of operation of the present invention. Nevertheless, as an aid to understanding and using the present invention, it should be noted that all conventional filter aid materials usable in preparing the materials of the present invention, as well as many other insoluble materials, develop negative electrostatic surface charges. This development of a negative surface charge is a phenomenon of the electrostatic double-layer effect in water; a discussion of this effect may be found in Jirgensons et al., A Short Textbook of Colloid Chemistry (New York, Wiley, 1954). Anion exchange resins have a positive surface charge, i.e., they are cationic particles, in water, and are therefore attracted to available, negatively charged surfaces. The magnitude of these attractive forces is small when compared to the mass of conventional ion exchange resins, but as the particle size of the resins is decreased, the force per unit weight increases. For particles with sizes near or below one micrometer, the forces are large enough to bond the particles to appropriate surfaces firmly enough that they will not dislodge under conditions typically used for filtration applications. When conventional filter aids are coated with cationic resin particles smaller than one micrometer in diameter, the cationic particles will not be removed from the filter aid surface by rinsing or use.